Integrated circuits typically are manufactured in processes that utilize pattern mask imaging. When these processes involve projection printing of the pattern mask images, radiant energy such as ultraviolet light is passed through the apertures of the pattern mask and is focused on a layer of resist that is coated on a substrate wafer. The light forms a pattern in the layer of resist that corresponds to the pattern, and the layer of resist is then developed to expose portions of the substrate to form a photoresist mask. Because the image of the pattern mask is brought into sharp focus on the photoresist layer surface, any contaminating particles present on the surface of the pattern mask are also brought into sharp focus on the surface of the photoresist layer, causing undesirable printing of the contaminating particle images in the resist layer. Printing of contaminating particle images on the photoresist layer often renders the photoresist mask useless, resulting in considerable economic waste.
To reduce the risk of particulate contamination of pattern masks, protective pellicles are generally used to cover the mask. Pattern mask pellicles include a transparent film that extends across one open surface of a frame of a size sufficient to enclose the pattern area of the mask, and a gasket that adheres the other open surface of the frame to the mask substrate. The frame separates the film from the pattern mask by a distance sufficient to keep the images of contaminating particles that are located on the outer surface of the pellicle out of focus, so that the images of the contaminating particles will not be printed with the circuit pattern on the light sensitive layer of resist.
Pellicles can be manufactured essentially particle-free and mounted over the pattern mask. The space over the mask pattern which is bounded by the mask substrate, gasket, frame and film is in excellently sealed condition hermetically. As a result, when the mask is transported by aircraft, transferred to a high altitude or otherwise subjected to a change in external pressure, the pressure difference between the pressure in the space over the mask pattern and the external pressure causes the pellicle film to bulge or become depressed. Such deformation of the film is disadvantageous in that not only the optical path of the assembly is changed, but also, depending on the degree of deformation, the film may come in contact with the surface of the mask or be damaged by contacting the surrounding package.
To avoid pellicle film deformation due to pressure differentials, different methods for pressure equalization have been proposed. Japanese patent publication JA 154061 and JA 22130 disclose a hole or holes straight through the frame to allow air to flow between the space over the pattern mask and the surrounding environment. The vent holes of both references compromise the essentially particle-free state of the space over the pattern mask.
In U.S. Pat. No. 4,833,051, Imamura also discloses vent holes in the pellicle frame. However, Imamura puts a filter in each vent hole. While the filter helps prevent contaminating particles from entering the space over the pattern mask through the vent hole, it also renders production of the pellicle more complex, difficult and expensive.